avcodec/jpeg2000dec: Check precno before using it in JPEG2000_PGOD_CPRL

Fixes out of array reads
Fixes: asan_heap-oob_f0de57_6823_mjp2.mov

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>